Method of manufacturing electrode comprising graphene layer on current collector

ABSTRACT

To provide a lithium-ion secondary battery having higher discharge capacity and higher energy density and a manufacturing method thereof. The lithium-ion secondary battery includes a positive electrode, a negative electrode, and an electrolyte provided between the positive electrode and the negative electrode. The positive electrode includes a positive electrode current collector and a positive electrode active material layer provided over the positive electrode current collector. In the positive electrode active material layer, graphenes and lithium-containing composite oxides are alternately provided. The lithium-containing composite oxide is a flat single crystal particle in which the length in the b-axis direction is shorter than each of the lengths in the a-axis direction and the c-axis direction. Further, the lithium-containing composite oxide is provided over the positive electrode current collector so that the b-axis of the single crystal particle intersects with a surface of the positive electrode current collector.

TECHNICAL FIELD

The present invention relates to a lithium-ion secondary battery and amanufacturing method thereof.

BACKGROUND ART

In recent years, lithium-ion secondary batteries have been developed.Because of their high thermal stability, lithium-containing compositeoxides having olivine structures, such as LiFePO₄, LiMnPO₄, LiCoPO₄, andLiNiPO₄, have been expected as positive electrode active materials oflithium-ion secondary batteries.

In order to increase the discharge capacity and the energy density oflithium-ion secondary batteries, attempts have been made to reduce theparticle diameters and variation in particle size of active materialsincluded in an active material layer that relates to intercalation anddeintercalation of ions functioning as carriers (see Patent Document 1).

REFERENCE

[Patent Document 1] PCT International Publication Ser. No. 08/077,447

DISCLOSURE OF INVENTION

However, lithium-containing composite oxides included in a lithium-ionsecondary battery have high resistance, so that there has been a limiton the increase of the discharge capacity and the energy density.

In view of the above problems, an object of one embodiment of thepresent invention is to provide a lithium-ion secondary battery havinghigher discharge capacity and higher energy density and a method formanufacturing such a lithium-ion secondary battery.

One embodiment of the present invention is a lithium-ion secondarybattery including a positive electrode, a negative electrode, and anelectrolyte provided between the positive electrode and the negativeelectrode. The positive electrode includes a positive electrode currentcollector and a positive electrode active material layer provided overthe positive electrode current collector. The positive electrode activematerial layer includes graphenes and lithium-containing compositeoxides. Specifically, in the positive electrode active material layer,the plurality of lithium-containing composite oxides is provided betweenthe different graphenes. The lithium-containing composite oxide isexpressed by a general formula LiMPO₄ (M is one or more of Fe(II),Mn(II), (Co(II), and Ni(I)). The lithium-containing composite oxide is aflat single crystal particle in which the length in the b-axis directionis shorter than each of the lengths in the a-axis direction and thec-axis direction. The length in the b-axis direction is typically longerthan or equal to 5 nm and shorter than or equal to 50 nm. Further, thelithium-containing composite oxide is provided over the positiveelectrode current collector so that the b-axis of the single crystalparticle intersects with a surface of the positive electrode currentcollector. Typically, the b-axis of the single crystal particleintersects with the surface of the positive electrode current collectorat any angle from 60° to 90°.

The lithium-containing composite oxides each have an olivine structure.The lithium-containing composite oxides each have an orthorhombiccrystal structure and belong to a space group Pnma (62). In each of thesingle crystal particles of the lithium-containing composite oxides, thelengths in the a-axis direction and the c-axis direction are each longerthan the length in the b-axis direction. The lithium-containingcomposite oxides may be stacked between the different graphenes.

The graphene refers to a sheet of one to ten atomic layers of carbonmolecules in which covalently-bonded carbon atoms form a six-memberedring which is a unit of repetition.

In the positive electrode of the lithium-ion secondary battery accordingto one embodiment of the present invention, the positive electrodeactive material layer includes an olivine-type lithium-containingcomposite oxide that is a flat single crystal particle in which thelength in the b-axis direction is shorter than each of the lengths inthe a-axis direction and the c-axis direction. Further, the b-axisintersects with the surface of the positive electrode current collector.Therefore, lithium ions are easily diffused between the currentcollector and the electrolyte. The use of the graphene for a conductionauxiliary agent allows an increase in proportion of a positive electrodeactive material in the positive electrode active material layer and areduction in resistance of the positive electrode active material layer.When the positive electrode includes the positive electrode activematerial layer having such a structure, the lithium-ion secondarybattery can have reduced internal resistance and higher power and can becharged and discharged at high speed. Moreover, the lithium-ionsecondary battery can have discharge capacity as high as theoreticaldischarge capacity.

According to one embodiment of the present invention, the dischargecapacity of a lithium-ion secondary battery can be increased, and thelithium-ion secondary battery can have higher power and can be chargedand discharged at high speed. Further, it is possible to manufacture alithium-ion secondary battery which has high discharge capacity and highpower and can be charged and discharged at high speed.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIGS. 1A to 1C illustrate positive electrodes of lithium-ion secondarybatteries;

FIG. 2 illustrates a crystal structure of olivine-type LiFePO₄;

FIGS. 3A to 3E illustrate a method for forming a positive electrode of alithium-ion secondary battery;

FIG. 4 illustrates a positive electrode and an electrolyte of alithium-ion secondary battery;

FIGS. 5A to 5E illustrate a method for manufacturing a lithium-ionsecondary battery;

FIG. 6 illustrates a lithium-ion secondary battery;

FIGS. 7A and 7B illustrate an application of a lithium-ion secondarybattery;

FIG. 8 illustrates an example of a structure of a wireless power feedingsystem; and

FIG. 9 illustrates an example of a structure of a wireless power feedingsystem.

BEST MODE FOR CARRYING OUT THE INVENTION

Examples of embodiments of the present invention will be described withreference to the drawings. Note that the present invention is notlimited to the following description, and it will be easily understoodby those skilled in the art that various changes and modifications canbe made without departing from the spirit and the scope of the presentinvention. Thus, the present invention should not be construed as beinglimited to the following description of the embodiments. In descriptionwith reference to the drawings, in some cases, common reference numeralsare used to denote the same portions in different drawings. Further, insome cases, the same hatching patterns are applied to similar portions,and the similar portions are not necessarily designated by referencenumerals.

Embodiment 1

In this embodiment, a positive electrode of a lithium-ion secondarybattery according to one embodiment of the present invention and amanufacturing method thereof will be described with reference to FIGS.1A to 1C, FIG. 2, and FIGS. 3A to 3E.

FIGS. 1A and 1B are each a cross-sectional view of a positive electrodeof a lithium-ion secondary battery.

As illustrated in FIG. 1A, graphenes 103 serving as a conductionauxiliary agent are provided over a positive electrode current collector101. Lithium-containing composite oxides 105 which are a positiveelectrode active material are provided over the graphenes 103. Graphenes113 serving as a conduction auxiliary agent are provided over thelithium-containing composite oxide 105. Lithium-containing compositeoxides 115 which are a positive electrode active material are providedover the graphenes 113. That is to say, the graphenes and thelithium-containing composite oxides are alternately stacked. Gapsbetween the graphenes 103, between the graphenes 113, between thelithium-containing composite oxides 105, and between thelithium-containing composite oxides 115 are filled with a binder 127.Note that the binder 127 is porous and fibrous and includes gaps; thus,when an electrolyte is a liquid electrolyte in the lithium-ion secondarybattery, the gaps between the graphenes 103 and between thelithium-containing composite oxides 105 which are the positive electrodeactive material are filled with the electrolyte.

Note that an electrolyte in this specification means the one whichincludes a material in which lithium ions stably exist and with whichlithium ions functioning as carrier ions can be transferred. Theelectrolyte includes in its category an electrolyte solution obtained bydissolving, in a solvent, a material (solute) in which lithium ionsstably exist, and a solid electrolyte including a material (solute) inwhich lithium ions stably exist, for example.

Note that the positive electrode active material refers to a materialthat relates to intercalation and deintercalation of ions which functionas carriers. Thus, the lithium-containing composite oxide is a positiveelectrode active material, whereas the graphene, the binder, a solvent,and the like are not positive electrode active materials.

As the positive electrode current collector 101, a material having highconductivity such as platinum, aluminum, copper, titanium, or stainlesssteel can be used. The positive electrode current collector 101 can havea foil shape, a plate shape, a net shape, or the like as appropriate.

The graphene 103 refers to a sheet of approximately one to ten atomiclayers of carbon molecules in which covalently-bonded carbon atoms forma six-membered ring which is a unit of repetition. Thus, the graphene103 is a pseudo two-dimensional sheet having a honeycomb structure. Inthe graphene 103, carbon atoms have sp² bonds.

Having significantly high carrier mobility at room temperature, thegraphene 103 can be used for a conduction auxiliary agent in thepositive electrode active material layer. Since the graphene 103 is asheet of approximately one to ten atomic layers of carbon moleculeshere, the volume thereof is extremely low; thus, the proportion of theconduction auxiliary agent included in a positive electrode activematerial layer 121 can be reduced, resulting in an increase inproportion of an active material in the positive electrode activematerial layer.

The desired thickness of the positive electrode active material layer121 is determined in the range of 20 μm to 100 μm. It is preferable toadjust the thickness of the positive electrode active material layer 121as appropriate so that a crack and separation are not caused.

The lithium-containing composite oxide 105 included in the positiveelectrode active material layer 121 is a single crystal particle havingan olivine structure. Typical examples of the olivine-typelithium-containing composite oxide (the general formula thereof isLiMPO₄ (M is one or more of Fe(II), Mn(II), Co(II), and Ni(II))) areLiFePO₄, LiNiPO₄, LiCoPO₄, LiMnPO₄, LiFe_(a)Ni_(b)PO₄,LiFe_(a)Co_(b)PO₄, LiFe_(a)Mn_(b)PO₄, LiNi_(a)Co_(b)PO₄,LiNi_(a)Mn_(b)PO₄ (a+b≤1, 0<a<1, and 0<b<1), LiFe_(c)Ni_(d)Co_(e)PO₄,LiFe_(c)Ni_(d)Mn_(e)PO₄, LiNi_(c)Co_(d)Mn_(e)PO₄ (c+d+e≤1, 0<c<1, 0<d<1,and 0<e<1), and LiFe_(f)Ni_(g)Co_(h)Mn_(i)PO₄ (f+g+h+i≤1, 0<f<1, 0<g<1,0<h<1, and 0<i<1).

Here, the shape of the lithium-containing composite oxide used in thisembodiment will be described with reference to FIG. 1C.

The lithium-containing composite oxide 105 has an orthorhombic crystalstructure and belongs to a space group Pnma (62). The lithium-containingcomposite oxide 105 is a flat crystal particle in which the length inthe b-axis direction is shorter than each of the lengths in the a-axisdirection and the c-axis direction. Since lithium ions are diffused inthe b-axis direction in an olivine structure, it is preferable to setthe length in the b-axis direction to longer than or equal to 5 nm andshorter than or equal to 50 nm, preferably longer than or equal to 5 nmand shorter than or equal to 20 nm so that lithium ions are easilydiffused. Further, it is preferable to set the ratio of the lengths inthe a-axis direction and the c-axis direction to greater than or equalto 0.5 and less than or equal to 1.5, preferably greater than or equalto 0.8 and less than or equal to 1.2, i.e., the b-plane having a squareshape or substantially square shape is preferable, because thelithium-containing composite oxides 105 can be arranged densely over thepositive electrode current collector 101.

As for the lithium-containing composite oxide, one or more of a side inthe a-axis direction, a side in the c-axis direction, and a planeincluding the side in the a-axis direction and the side in the c-axisdirection, i.e., the b-plane are in contact with the graphene 103, andthe b-axis of the single crystal particle intersects with a surface ofthe positive electrode current collector 101. The b-axis of thelithium-containing composite oxide intersects with the surface of thepositive electrode current collector 101 typically at any angle from 60°to 90°. Since lithium ions are diffused in the b-axis direction in anolivine structure, it is preferable that the b-axis intersect with thesurface of the positive electrode current collector 101 at any anglefrom 60° to 90° to diffuse a larger number of lithium ions. Note thatthe term “the b-axis intersects with the surface of the positiveelectrode current collector 101” means that the b-axis and the surfaceof the positive electrode current collector 101 have an intersectionpoint. In contrast, the term “the b-axis does not intersect with thesurface of the positive electrode current collector 101” means that theb-axis is in parallel to the surface of the positive electrode currentcollector 101.

Note that it can be judged using more than one of a scanning electronmicroscope (SEM), a scanning transmission electron microscope (STEM), atransmission electron microscope (TEM), and X-ray diffraction (XRD) thatthe lithium-containing composite oxide 105 is a flat crystal in whichthe length of the side in the b-axis direction is shorter than each ofthe lengths of the sides in the a-axis direction and the c-axisdirection. For example, it can be judged by X-ray diffraction (XRD)measurement that the b-axis of the single crystal particle of thelithium-containing composite oxide 105 intersects with the surface ofthe positive electrode current collector 101. Further, thelithium-containing composite oxide 105 is judged as a single crystalparticle because the contrast of a dark-field image observed with atransmission electron microscope (TEM) is uniform and thus grainboundaries are not seen in the dark-field image.

Here, description is given of an olivine structure. FIG. 2 illustrates aunit cell 301 of lithium iron phosphate (LiFePO₄) that is an example ofan olivine-type lithium-containing composite oxide. An olivine-typelithium iron phosphate has an orthorhombic crystal structure andincludes tour formula units of lithium iron phosphate (LiFePO₄) within aunit cell. The basic framework of the olivine structure is a hexagonalclosest packed structure of oxide ions, in which lithium, iron, andphosphorus are located in gaps of the closest packed structure.

Further, the olivine-type lithium iron phosphate (LiFePO₄) has atetrahedral site and two kinds of octahedral sites. The tetrahedral sitehas four oxygen atoms in the vertices. The octahedral sites have sixoxygen atoms in the vertices. Phosphorus 307 is located at the center ofthe tetrahedral site, and lithium 303 or iron 305 is located at thecenter of the octahedral site. The octahedral site with the lithium 303located at the center is referred to as an M1 site, and the octahedralsite with the iron 305 located at the center is referred to as an M2site. The M1 site is disposed one-dimensionally in the b-axis direction.In other words, the lithium 303 is disposed one-dimensionally in the<010> direction. Note that for sake of simplicity, the bonds between thelithium 303 and other ions or atoms are not shown by lines.

The irons 305 of neighboring M2 sites are bonded in a zigzag manner withoxygen 309 interposed therebetween. The oxygen 309 bonded between theirons 305 of the neighboring M2 sites is also bonded to the phosphorus307 of the tetrahedral site. Thus, the bonds of iron-oxygen-phosphorusare serially linked.

Note that the olivine-type lithium iron phosphate may be distorted.Furthermore, regarding the lithium iron phosphate, the composition ratioof lithium, iron, phosphorus, and oxygen is not limited to 1:1:1:4.Also, as the transition metal (M) of a lithium transition metalphosphate (LiMPO₄), a transition metal which has a larger ionic radiusthan a lithium ion, such as manganese, cobalt, or nickel, may be used.

When lithium is deintercalated from the olivine-type lithium ironphosphate in FIG. 2, iron phosphate is left, and this iron phosphate hasa stable structure. Thus, intercalation and deintercalation of alllithium ions are possible. Further, the olivine-type lithium ironphosphate has thermal stability. In the olivine-type lithium ironphosphate, lithium ions are unidimensionally arranged in the b-axisdirection and diffused in the b-axis direction. For this reason, whenthe length of the side in the b-axis direction of the single crystalparticle is short, the lithium ions can be easily diffused.

In the positive electrode according to this embodiment, the positiveelectrode active material layer includes an olivine-typelithium-containing composite oxide which is a flat single crystalparticle whose length in the b-axis direction is shorter than each ofthe lengths in the a-axis direction and the c-axis direction. Further,one or more of the side in the a-axis direction, the side in the c-axisdirection, and a plane including the side in the a-axis direction andthe side in the c-axis direction, i.e., the b-plane are in contact withgraphene having high conductivity, and the b-axis whose direction is thedirection of diffusion of lithium ions in the olivine structureintersects with the surface of the positive electrode current collector.Therefore, a larger number of lithium ions can be diffused between thecurrent collector and an electrolyte.

The use of graphene for the conduction auxiliary agent allows areduction in proportion of the conduction auxiliary agent in thepositive electrode active material layer and a reduction in resistanceof the positive electrode active material layer. Further, film-likegraphene which is the conduction auxiliary agent and thelithium-containing composite oxides are alternately stacked and thelithium-containing composite oxides each have a flat shape; thus, it ispossible to increase the fill rate of the lithium-containing compositeoxides in the positive electrode active material layer. That is to say,the proportion of the positive electrode active material in the positiveelectrode active material layer can be increased and the resistance ofthe positive electrode active material layer can be reduced.Accordingly, when the positive electrode active material layer describedin this embodiment is used for the positive electrode, the lithium-ionsecondary battery can have reduced internal resistance and higher powerand can be charged and discharged at high speed. Moreover, thelithium-ion secondary battery can have discharge capacity as high astheoretical discharge capacity.

As illustrated in FIG. 1B, a plurality of lithium-containing compositeoxides may be stacked in a positive electrode active material layer 141.Specifically, graphenes 123 are provided over the positive electrodecurrent collector 101, and a plurality of lithium-containing compositeoxides 125 is stacked over the graphenes 123. In addition, graphenes 133are provided over the lithium-containing composite oxides 125, and aplurality of lithium-containing composite oxides 135 are stacked overthe graphene 133. The use of the positive electrode in FIG. 1B leads toa further increase in discharge capacity of the lithium-ion secondarybattery as compared with the case of using the positive electrode inFIG. A.

Note that in FIGS. 1A and 1B, graphenes may be provided over surfaces ofthe lithium-containing composite oxides 115 and 135. The graphenes 103and 123 are not necessarily provided over the positive electrode currentcollector 101, and the lithium-containing composite oxides 105 and 125may be in contact with the positive electrode current collector 101.

Next, a method for forming the positive electrode of the lithium-ionsecondary battery illustrated in FIG. 1A will be described withreference to FIGS. 3A to 3E.

As illustrated in FIG. 3A, the graphenes 103 are provided as theconduction auxiliary agent over the positive electrode current collector101. The graphene 103 can be formed by epitaxial growth on foil or afilm of a transition metal, a coating method, a chemical separationmethod, or the like.

A process of epitaxial growth on foil or a film of a transition metal isas follows. Foil of a transition metal serving as a catalyst, such asnickel or iron, is formed over a substrate, the substrate is placed in achamber and heated to 600° C. to 1100° C., inclusive, and a gascontaining hydrocarbon such as methane or ethane is introduced into thechamber, so that graphene is formed over the substrate. Then, the foilof the transition metal is etched with an acid solution or the like toobtain the graphene. Note that instead of the substrate provided withthe foil of the transition metal, a film of a transition metal may beused.

A coating method is as follows. A sulfuric acid solution of potassiumpermanganate, oxygenated water, or the like is mixed into single crystalgraphite powder to cause oxidation reaction; thus, a graphene oxideaqueous solution is formed. Then, the graphene oxide aqueous solution isapplied onto an appropriate substrate provided with a separation layerand dried. As the separation layer, a metal film which has a thicknessfrom 1 nm to 100 nm and is soluble in an acid solution may be used.Then, graphene oxide is reduced by high-temperature heating in vacuum,addition of a reducing agent such as hydrazine, or the like, so thatgraphene is formed. After that, the separation layer is etched with anacid solution or the like, whereby graphene is obtained.

In the case of using a reducing agent in the manufacturing method,reduction reaction proceeds from a surface; therefore, the reductionreaction can be terminated at an appropriate depth by controllingreaction time. In this state, reduced graphene is obtained at thesurface, while graphene oxide remains in an unreacted portion. Sincegraphene oxide can be dissolved in water, when the substrate is soakedin water, graphene insoluble in water can be obtained. The grapheneoxide dissolved in water can be collected and applied onto anothersubstrate.

A chemical separation method is a method in which graphene is chemicallyseparated from graphite. Typically, graphite is placed in a polarsolvent such as chloroform, N,N-dimethylformamide (DMF), orN-methylpyrrolidone (NMP) and bonds between layers in the graphite arebroken by ultrasonic vibration, so that graphene can be obtained.

Next, as illustrated in FIG. 3B, slurry 109 including thelithium-containing composite oxides 105 is applied to the positiveelectrode current collector 101 and the graphenes 103. Then, it ispreferable to make the thickness of the slurry 109 including thelithium-containing composite oxides 105 uniform or substantially uniformwith a squeegee, a blade, or the like. Further, a solvent of the slurry109 may be dried to increase the viscosity of the slurry 109. In thisstep, as illustrated in FIG. 3B, the lithium-containing composite oxides105 are applied randomly to the positive electrode current collector 101or the graphenes 103, thus, the a-axis, the b-axis, and the c-axis ofthe lithium-containing composite oxides 105 intersect with the surfaceof the positive electrode current collector 101. The lithium-containingcomposite oxide 105 is a flat single crystal particle in which thelength in the b-axis direction is shorter than each of the lengths inthe a-axis direction and the c-axis direction. Therefore, when thelithium-containing composite oxides 105 are dispersed over the positiveelectrode current collector 101 so that the a-axes or the c-axes of thelithium-containing composite oxides 105 intersect with the surface ofthe positive electrode current collector 101 or the graphene 103, i.e.,so that the a-planes or the c-planes of the lithium-containing compositeoxides 105 are in contact with the positive electrode current collector101 or the graphene 103, the lithium-containing composite oxides lie sothat the height thereof is high as shown by a lithium-containingcomposite oxide 105 a.

The slurry 109 including the lithium-containing composite oxides furtherincludes a binder, the solvent, and the like.

A solid phase method, a hydrothermal method, a spray pyrolysis method,or the like can be used as appropriate for forming thelithium-containing composite oxides. Note that a hydrothermal method ispreferably employed for manufacturing flat single crystal particleswhich have small particle diameters and less variation in particle sizeand in each of which the length in the b-axis direction is shorter thaneach of the lengths in the a-axis direction and the c-axis direction.

As the binder, polysaccharides such as starch, carboxymethyl cellulose,hydroxypropyl cellulose, regenerated cellulose, and diacetyl cellulose;vinyl polymers such as polyvinyl chloride, polyethylene, polypropylene,polyvinyl alcohol, polyvinyl pyrrolidone, polytetrafluoroethylene,polyvinylide fluoride, ethylene-propylene-diene monomer (EPDM) rubber,sulfonated EPDM rubber, styrene-butadiene rubber, butadiene rubber, andfluorine rubber; polyether such as polyethylene oxide; and the like canbe given.

Note that a solvent may be used as appropriate to disperse or dissolvethe lithium-containing composite oxides and the hinder in the slurry.

Lithium-containing composite oxides with small particle diameters arelikely to agglomerate and difficult to disperse uniformly in the slurry.For this reason, a dispersant and a disperse medium are preferably usedas appropriate to disperse the lithium-containing composite oxidesuniformly in the slurry.

As the dispersant, a high-molecular dispersant, a surfactant dispersant(low-molecular dispersant), an inorganic dispersant, and the like aregiven. As the disperse medium, alcohol, water, and the like are given.Note that the dispersant and the disperse medium may be selected asappropriate depending on the lithium-containing composite oxide.

Next, physical pressure is exerted on the slurry 109 including thelithium-containing composite oxides 105. As a method for exertingphysical pressure on the slurry 109 including the lithium-containingcomposite oxides 105, a method in which a roller, a squeegee, a blade,or the like is moved on the slurry 109 including the lithium-containingcomposite oxides 105 is given. Alternatively, ultrasonic vibration maybe transmitted to the slurry including the lithium-containing compositeoxides, instead of exerting physical pressure on the slurry.Consequently, in the slurry 109 including the lithium-containingcomposite oxides 105, the lithium-containing composite oxide 105 whosea-axis or c-axis intersects with the surface of the positive electrodecurrent collector 101, i.e., the lithium-containing composite oxide 105a whose a-plane or c-plane is in contact with the graphene 103 fallsdown; thus, the lithium-containing composite oxide 105 can be in thestate where the b-axis thereof intersects with the surface of thepositive electrode current collector 101. In addition, one or more ofthe side in the a-axis direction, the side in the c-axis direction, andthe b-plane of the lithium-containing composite oxide 105 can be incontact with the graphene 103. In other words, the area of regions ofthe graphenes 103 which are in contact with the lithium-containingcomposite oxides 105 can be increased.

Then, the slurry 109 including the lithium-containing composite oxides105 is heated to remove the solvent and to fix the lithium-containingcomposite oxides 105 with a binder 107 (see FIG. 3C). The binder becomesporous and fibrous through the heating and includes gaps 108, so thatthe lithium-containing composite oxides are exposed in the gaps.

Alternatively, the positive electrode current collector 101 over whichthe graphenes 103 are formed is soaked in the slurry 109 including thelithium-containing composite oxides 105 and then is gradually lifted.After that, the slurry 109 is heated to remove the solvent and to fixthe lithium-containing composite oxides 105 with the binder 107.Consequently, the lithium-containing composite oxides 105 can be in thestate where the b-axes thereof intersect with the surface of thepositive electrode current collector 101 as illustrated in FIG. 3C. Inthat case, the positive electrode current collector 101 is graduallylifted after the surface tension between the slurry 109 and the positiveelectrode current collector 101 or the graphene 103 is controlled sothat the meniscus of the slurry 109 is concave. Capillary action at endportions of the meniscus enables the state where the b-axes of thelithium-containing composite oxides 105 intersect with the surface ofthe positive electrode current collector 101 as illustrated in FIG. 3C.

Next, as illustrated in FIG. 3D, the graphenes 113 are formed over thelithium-containing composite oxides 105 in a manner similar to that inFIG. 3A.

Then, by performing the steps in FIGS. 3B and 3C, the lithium-containingcomposite oxides 115 and a binder 117 are formed over the graphenes 113as illustrated in FIG. 3E.

Through the above steps, the positive electrode of the lithium-ionsecondary battery, in which the positive electrode active material layer121 where the lithium-containing composite oxides 105 and 115 and thegraphenes 103 and 113 are alternately stacked is provided over thepositive electrode current collector 101, can be manufactured. Note thatin the positive electrode active material layer 121, the binders 107 and117 are collectively referred to as the binder 127.

Although FIGS. 3A to 3E illustrate the manufacturing method of apositive electrode of a lithium-ion secondary battery, in which onelayer of lithium-containing composite oxides is provided between thegraphenes 103 and 113 in FIG. 1A, when a plurality of layers oflithium-containing composite oxides are provided between graphenes, thepositive electrode active material layer 141 in which thelithium-containing composite oxides 125 are stacked between thegraphenes 123 and 133 as illustrated in FIG. 1B can be manufactured.

In the positive electrode according to this embodiment, the positiveelectrode active material layer includes an olivine-typelithium-containing composite oxide which is a flat single crystalparticle whose length in the b-axis direction is shorter than each ofthe lengths in the a-axis direction and the c-axis direction. Further,one or more of the side in the a-axis direction, the side in the c-axisdirection, and a plane including the side in the a-axis and the side inthe c-axis, i.e., the b-plane are in contact with the graphene, and theb-axis whose direction is the direction of diffusion of lithium ionsintersects with the surface of the positive electrode current collector.Therefore, a larger number of lithium ions can be diffused between thecurrent collector and an electrolyte. Further, the use of graphene forthe conduction auxiliary agent allows a reduction in proportion of theconduction auxiliary agent in the positive electrode active materiallayer and a reduction in resistance of the positive electrode active,material layer. That is to say, the proportion of the positive electrodeactive material in the positive electrode active material layer can beincreased and the resistance of the positive electrode active materiallayer can be reduced. Accordingly, when the positive electrode describedin this embodiment is used for the lithium-ion secondary battery, thelithium-ion secondary battery can have reduced internal resistance andhigher power and can be charged and discharged at high speed. Moreover,the lithium-ion secondary battery can have discharge capacity as high astheoretical discharge capacity.

Embodiment 2

In this embodiment, a method for forming a positive electrode includinga positive electrode current collector and a positive electrode activematerial layer in which gaps are filled with a solid electrolyte overthe positive current collector will be described.

In this embodiment, in the positive electrode active material layerdescribed in Embodiment 1, the binder includes a solute of anelectrolyte of a lithium-ion secondary battery.

FIG. 4 illustrates the positive electrode according to this embodiment.

As illustrated in FIG. 4, the graphenes 103 and the lithium-containingcomposite oxides 105 are stacked in this order over the positiveelectrode current collector 101. Further, the graphenes 113 are providedover the lithium-containing composite oxides 105, and thelithium-containing composite oxides 115 are stacked over the graphenes113. Furthermore, a binder 187 including a solute of an electrolyte andserving as a solid electrolyte of a lithium-ion secondary battery isprovided in gaps between the graphenes 103, between the graphenes 113,between the lithium-containing composite oxides 105, and between thelithium-containing composite oxides 115. A region where the graphenes103 and 113 and the lithium-containing composite oxides 105 and 115 arestacked functions as a positive electrode active material layer 161.

As a solute of the electrolyte, a material in which lithium ions thatare carrier ions can transfer and stably exist is used. Typical examplesof the solute of the electrolyte include lithium salts such as LiClO₄,LiAsF₆, LiBF₄, LiPF₆, and Li(C₂F₅SO₂)₂N.

Note that a solvent may be used as appropriate to disperse or dissolvethe solute of the electrolyte in slurry.

Next, methods for forming the positive electrode and the electrolytewhich are illustrated in FIG. 4 will be described with reference toFIGS. 5A to 5E.

In a manner similar to that in Embodiment 1, slurry 149 including thesolute of the electrolyte as well as the lithium-containing compositeoxides 105 and the binder is applied to the graphenes 103 formed overthe positive electrode current collector 101. Then, the thickness of theslurry 149 may be made uniform and a solvent of the slurry 149 may bedried.

Then, in a manner similar to that in Embodiment 1, physical pressure isexerted on the slurry 149 including the lithium-containing compositeoxides 105. Alternatively, ultrasonic vibration may be transmitted tothe slurry 149 including the lithium-containing composite oxides 105.Still alternatively, after being soaked in the slurry 149 including thelithium-containing composite oxides 105, the positive electrode currentcollector 101 over which the graphenes 103 are formed may be graduallylifted. Consequently, the lithium-containing composite oxides 105 can bein the state where the b-axes thereof intersect with the surface of thepositive electrode current collector 101 as illustrated in FIG. 5A. Inaddition, one or more of the side in the a-axis direction, the side inthe c-axis direction, and the b-plane of the lithium-containingcomposite oxide 105 can be in contact with the graphene 103.

Then, in a manner similar to that in Embodiment 1, the slurry 149including the lithium-containing composite oxides 105 is heated toremove the solvent and to fix the lithium-containing composite oxides105 with a binder 147 including the electrolyte (see FIG. 5B).

Next, as illustrated in FIG. 5C, the graphenes 113 are formed over thelithium-containing composite oxides 105 in a manner similar to that inEmbodiment 1.

Then, by performing the steps in FIGS. 5A and 5B, the lithium-containingcomposite oxides 115 and a binder 157 are formed over the graphenes 113as illustrated in FIG. 5D. By heat treatment, gaps 118 are formed in thebinder 157; thus, the lithium-containing composite oxides 115 areexposed in the gaps 118.

Through the above steps, the positive electrode active material layer161 where the lithium-containing composite oxides 105 and 115 and thegraphenes 103 and 113 are alternately stacked can be provided over thepositive electrode current collector 101.

Although FIGS. 5A to 5E illustrate the manufacturing method of thepositive electrode in which one layer of lithium-containing compositeoxides is provided between the graphenes 103 and 113, a plurality oflayers of lithium-containing composite oxides may be provided betweenthe graphenes 103 and 113, as appropriate. The graphenes 103 are notnecessarily provided over the positive electrode current collector 101,and the lithium-containing composite oxides 105 may be in contact withthe positive electrode current collector 101.

After that, a binder 167 including the solute of the electrolyte of thelithium-ion secondary battery may be provided over the positiveelectrode active material layer 161 (see FIG. 5E). Through the abovesteps, the positive electrode active material layer 161 in which gapsare filled with the solid electrolyte can be formed. Note that in FIG.5E, the binders 147, 157, and 167 each including the solute of theelectrolyte of the lithium-ion secondary battery are collectivelyreferred to as a binder 187 including the solute of the electrolyte ofthe lithium-ion secondary battery.

According to this embodiment, a positive electrode in which a positiveelectrode active material layer filled with a solid electrolyte isprovided over a positive electrode current collector can be formed;thus, the resistance at the interface between the electrode and theelectrolyte can be reduced. Accordingly, with the use of the positiveelectrode described in this embodiment, the internal resistance of alithium-ion secondary battery is further reduced, the lithium-ionsecondary battery can have higher power and can be charged anddischarged at high speed, and the discharge capacity can be as high astheoretical discharge capacity.

Embodiment 3

In this embodiment, a lithium-ion secondary battery and a manufacturingmethod thereof will be described.

A lithium-ion secondary battery according to this embodiment will bedescribed with reference to FIG. 6. Here, a cross-sectional structure ofthe lithium-ion secondary battery will be described below.

FIG. 6 is a cross-sectional view of the lithium-ion secondary battery.

A lithium-ion secondary battery 400 includes a negative electrode 411including a negative electrode current collector 407 and a negativeelectrode active material layer 409, a positive electrode 405 includinga positive electrode current collector 401 and a positive electrodeactive material layer 403, and a separator 413 provided between thenegative electrode 411 and the positive electrode 405. Note that theseparator 413 is impregnated with an electrolyte. The negative electrodecurrent collector 407 is connected to an external terminal 419 and thepositive electrode current collector 401 is connected to an externalterminal 417. An end portion of the external terminal 419 is embedded ina gasket 421. That is to say, the external terminals 417 and 419 areinsulated from each other by the gasket 421.

For the negative electrode current collector 407, a material having highconductivity such as copper, stainless steel, iron, or nickel can beused. The negative electrode current collector 407 can have a foilshape, a plate shape, a net shape, or the like as appropriate.

The negative electrode active material layer 409 is formed using amaterial capable of lithium-ion occlusion and emission. Typically,lithium, aluminum, graphite, silicon, tin, germanium, or the like isused. Note that it is possible to omit the negative electrode currentcollector 407 and use the negative electrode active material layer 409alone for a negative electrode. The theoretical lithium occlusioncapacity is larger in germanium, silicon, lithium, and aluminum than ingraphite. When the occlusion capacity is large, charge and discharge canbe performed sufficiently even in a small area and a function of anegative electrode can be obtained, so that reduction in cost and sizeof a lithium-ion secondary battery can be achieved. However, in the caseof silicon or the like, the volume is approximately quadrupled due tolithium occlusion; therefore, the probability that the material itselfgets vulnerable should be considered.

Note that the negative electrode active material layer 409 may bepredoped with lithium. Predoping with lithium may be performed in such amanner that a lithium layer is formed on a surface of the negativeelectrode active material layer 409 by a sputtering method.Alternatively, lithium foil is provided on the surface of the negativeelectrode active material layer 409, whereby the negative electrodeactive material layer 409 can be predoped with lithium.

The desired thickness of the negative electrode active material layer409 is determined in the range of 20 μm to 100 μm.

Note that the negative electrode active material layer 409 may include abinder and a conduction auxiliary agent. As the binder and theconduction auxiliary agent, the binder and the conduction auxiliaryagent which are included in the positive electrode active material layerdescribed in Embodiment 1 can be used as appropriate.

As the positive electrode current collector 401 and the positiveelectrode active material layer 403, the positive electrode currentcollector 101 and the positive electrode active material layer 121 or141 which are described in Embodiment 1 can be used as appropriate.

As the separator 413, an insulating porous material is used. Typicalexamples of the separator 413 include cellulose (paper), polyethylene,polypropylene, and the like.

As a solute of the electrolyte, such a material described in Embodiment2, in which lithium ions, which are carrier ions, can transfer andstably exist, is used as appropriate.

As a solute of the electrolyte, a material in which lithium ions thatare carrier ions can transfer and exist stably is used. As the solventof the electrolyte, a material in which lithium ions can transfer isused. As the solvent of the electrolyte, an aprotic organic solvent ispreferably used. Typical examples of aprotic organic solvents includeethylene carbonate, propylene carbonate, dimethyl carbonate, diethylcarbonate, γ-butyrolactone, acetonitrile, dimethoxyethane,tetrahydrofuran, and the like, and one or more of these materials can beused. When a gelled high-molecular material is used as the solvent ofthe electrolyte, safety against liquid leakage and the like is improved.Further, the lithium-ion secondary battery 400 can be thinner and morelightweight. Typical examples of gelled high-molecular materials includea silicon gel, an acrylic gel, an acrylonitrile gel, polyethylene oxide,polypropylene oxide, a fluorine-based polymer, and the like.

As the electrolyte, a solid electrolyte such as Li₃PO₄ can be used. Notethat in the case of using such a solid electrolyte as the electrolyte,the separator 413 is unnecessary.

Instead of the positive electrode and the electrolyte, a solidelectrolyte including a positive electrode active material, which isformed over the positive electrode current collector as described inEmbodiment 2, may be used.

For the external terminals 417 and 419, a metal member such as astainless steel plate or an aluminum plate can be used as appropriate.

Note that in this embodiment, a coin-type lithium-ion secondary batteryis given as the lithium-ion secondary battery 400; however, any oflithium-ion secondary batteries with various shapes, such as asealing-type lithium-ion secondary battery, a cylindrical lithium-ionsecondary battery, and a square-type lithium-ion secondary battery, canbe used. Further, a structure in which a plurality of positiveelectrodes, a plurality of negative electrodes, and a plurality ofseparators are stacked or rolled may be employed.

A lithium-ion secondary battery has a high energy density, a largecapacity, and a high output voltage, which enables reduction in size andweight. Further, the lithium-ion secondary battery does not easilydeteriorate due to repetitive charge and discharge and can be used for along time, so that cost can be reduced. When olivine-typelithium-containing composite oxides each of which is a flat singlecrystal particle whose length in the h-axis direction is shorter thaneach of the lengths in the a-axis direction and the c-axis direction andgraphenes having high conductivity are alternately stacked in thepositive electrode active material layer, the lithium-ion secondarybattery can have higher discharge capacity and higher power and can becharged and discharged at high speed.

Next, a method for manufacturing the lithium-ion secondary battery 400according to this embodiment will be described.

First, a method for forming the negative electrode 411 will bedescribed.

The negative electrode active material layer 409 is formed over thenegative electrode current collector 407 by a coating method, asputtering method, an evaporation method, or the like, whereby thenegative electrode 411 can be formed. Alternatively, for the negativeelectrode 411, foil, a plate, or mesh of lithium, aluminum, graphite, orsilicon can be used. Here, graphite is predoped with lithium to form thenegative electrode.

Next, the method for forming the positive electrode described inEmbodiment 1 is employed as appropriate to form the positive electrode405.

Next, the negative electrode 411, the separator 413, and the positiveelectrode 405 are impregnated with the electrolyte. Then, the positiveelectrode 405, the separator 413, the gasket 421, the negative electrode411, and the external terminal 419 are stacked in this order over theexternal terminal 417, and the external terminal 417 and the externalterminal 419 are crimped to each other with a “coin cell crimper”. Thus,the coin-type lithium-ion secondary battery can be manufactured.

Note that a spacer and a washer may be provided between the externalterminal 417 and the positive electrode 405 or between the externalterminal 419 and the negative electrode 411 so that the connectionbetween the external terminal 417 and the positive electrode 405 orbetween the external terminal 419 and the negative electrode 411 isenhanced.

Embodiment 4

In this embodiment, an application of the lithium-ion secondary batterydescribed in Embodiment 3 will be described with reference to FIGS. 7Aand 7B.

The lithium-ion secondary battery described in Embodiment 3 can beprovided in electronic devices, e.g., cameras such as digital cameras orvideo cameras, digital photo frames, mobile phones (also referred to ascellular phones or cellular phone devices), portable game machines,portable information terminals, audio reproducing devices, and the like.Moreover, the lithium-ion secondary battery can be provided inelectrically propelled vehicles such as electric vehicles, hybridvehicles, electric railway cars, service vehicles, carts, and electricwheelchairs. Here, examples of the electrically propelled vehicles willbe described.

FIG. 7A illustrates a structure of a four-wheeled automobile 500 as anexample of the electrically propelled vehicles. The automobile 500 is anelectric vehicle or a hybrid vehicle. An example is illustrated in whichthe automobile 500 is provided with a lithium-ion secondary battery 502on its bottom portion. In order to clearly show the position of thelithium-ion secondary battery 502 in the automobile 500, FIG. 7B showsthe outline of the automobile 500 and the lithium-ion secondary battery502 provided on the bottom portion of the automobile 500. Thelithium-ion secondary battery described in Embodiment 3 can be used asthe lithium-ion secondary battery 502. The lithium-ion secondary battery502 can be charged by being externally supplied with electric power by aplug-in technique or a wireless power feeding system.

Embodiment 5

In this embodiment, examples of using a lithium-ion secondary batteryaccording to one embodiment of the present invention in a wireless powerfeeding system (hereinafter referred to as an RF power feeding system)will be described with reference to block diagrams in FIG. 8 and FIG. 9.In each of the block diagrams, blocks show elements independently, whichare classified according to their functions, within a power receivingdevice and a power feeding device. However, it is practically difficultto completely separate the elements according to their functions; insome cases, one element can involve a plurality of functions.

First, the RF power feeding system will be described with reference toFIG. 8.

A power receiving device 600 is an electronic device or an electricallypropelled vehicle which is driven by electric power supplied from apower feeding device 700, and can be applied to any other devices whichare driven by electric power, as appropriate. Typical examples of theelectronic device include cameras such as digital cameras or videocameras, digital photo frames, mobile phones, portable game machines,portable information terminals, audio reproducing devices, displaydevices, computers, and the like. Typical examples of the electricallypropelled vehicle include electric vehicles, hybrid vehicles, electricrailway cars, service vehicles, carts, electric wheelchairs, and thelike. In addition, the power feeding device 700 has a function ofsupplying electric power to the power receiving device 600.

In FIG. 8, the power receiving device 600 includes a power receivingdevice portion 601 and a power load portion 610. The power receivingdevice portion 601 includes at least a power receiving device antennacircuit 602, a signal processing circuit 603, and a lithium-ionsecondary battery 604. The power feeding device 700 includes at least apower feeding device antenna circuit 701 and a signal processing circuit702.

The power receiving device antenna circuit 602 has a function ofreceiving a signal transmitted by the power feeding device antennacircuit 701 and a function of transmitting a signal to the power feedingdevice antenna circuit 701. The signal processing circuit 603 processesa signal received by the power receiving device antenna circuit 602 andcontrols charging of the lithium-ion secondary battery 604 and supplyingof electric power from the lithium-ion secondary battery 604 to thepower load portion 610. In addition, the signal processing circuit 603controls operation of the power receiving device antenna circuit 602.That is, the signal processing circuit 603 can control the intensity,the frequency, or the like of a signal transmitted by the powerreceiving device antenna circuit 602. The power load portion 610 is adrive portion which receives electric power from the lithium-ionsecondary battery 604 and drives the power receiving device 600. Typicalexamples of the power load portion 610 include a motor, a drivercircuit, and the like. Another device which receives electric power anddrives the power receiving device may be used as the power load portion610 as appropriate. The power feeding device antenna circuit 701 has afunction of transmitting a signal to the power receiving device antennacircuit 602 and a function of receiving a signal from the powerreceiving device antenna circuit 602. The signal processing circuit 702processes a signal received by the power feeding device antenna circuit701. In addition, the signal processing circuit 702 controls operationof the power feeding device antenna circuit 701. That is, the signalprocessing circuit 702 can control the intensity, the frequency, or thelike of a signal transmitted by the power feeding device antenna circuit701.

The lithium-ion secondary battery according to one embodiment of thepresent invention is used as the lithium-ion secondary battery 604included in the power receiving device 600 in the RF power feedingsystem shown in FIG. 8.

When the lithium-ion secondary battery according to one embodiment ofthe present invention is used in the RF power feeding system, thedischarge capacity or the charge capacity (also referred to as theamount of power storage) can be increased as compared with the case ofusing a conventional lithium-ion secondary battery. Therefore, the timeinterval between wireless power feeding and the next wireless powerfeeding can be longer (power feeding can be less frequent).

In addition, with the use of the lithium-ion secondary battery accordingto one embodiment of the present invention in the RF power feedingsystem, the power receiving device 600 can be compact and lightweight ifthe discharge capacity or the charge capacity with which the power loadportion 610 can be driven is the same as that of a conventionalsecondary battery. Therefore, the total cost can be reduced.

Next, another example of the RF power feeding system will be describedwith reference to FIG. 9.

In FIG. 9, the power receiving device 600 includes the power receivingdevice portion 601 and the power load portion 610. The power receivingdevice portion 601 includes at least the power receiving device antennacircuit 602, the signal processing circuit 603, the lithium-ionsecondary battery 604, a rectifier circuit 605, a modulation circuit606, and a power supply circuit 607. In addition, the power feedingdevice 700 includes at least the power feeding device antenna circuit701, the signal processing circuit 702, a rectifier circuit 703, amodulation circuit 704, a demodulation circuit 705, and an oscillatorcircuit 706.

The power receiving device antenna circuit 602 has a function ofreceiving a signal transmitted by the power feeding device antennacircuit 701 and a function of transmitting a signal to the power feedingdevice antenna circuit 701. In the case where the power receiving deviceantenna circuit 602 receives a signal transmitted by the power feedingdevice antenna circuit 701, the rectifier circuit 605 generates DCvoltage from the signal received by the power receiving device antennacircuit 602. The signal processing circuit 603 has a function ofprocessing a signal received by the power receiving device antennacircuit 602 and a function of controlling charging of the lithium-ionsecondary battery 604 and supply of electric power from the lithium-ionsecondary battery 604 to the power supply circuit 607. The power supplycircuit 607 has a function of converting voltage stored in thelithium-ion secondary battery 604 into voltage needed for the power loadportion 610. The modulation circuit 606 is used when a certain responseis transmitted from the power receiving device 600 to the power feedingdevice 700.

With the power supply circuit 607, electric power to be supplied to thepower load portion 610 can be controlled. Thus, overvoltage applicationto the power load portion 610 can be suppressed, leading to suppressionof deterioration or breakdown of the power receiving device 600.

In addition, provision of the modulation circuit 606 enablestransmission of a signal from the power receiving device 600 to thepower feeding device 700. Therefore, when it is judged from the amountof charge of the power receiving device 600 that a certain amount ofpower is stored, a signal is transmitted from the power receiving device600 to the power feeding device 700 so that power feeding from the powerfeeding device 700 to the power receiving device 600 can be stopped. Asa result, the lithium-ion secondary battery 604 is not fully charged, sothat the number of charge cycles of the lithium-ion secondary battery604 can be increased.

The power feeding device antenna circuit 701 has a function oftransmitting a signal to the power receiving device antenna circuit 602and a function of receiving a signal from the power receiving deviceantenna circuit 602. When a signal is transmitted to the power receivingdevice antenna circuit 602, the signal processing circuit 702 generatesa signal to be transmitted to the power receiving device. The oscillatorcircuit 706 is a circuit which generates a signal with a constantfrequency. The modulation circuit 704 has a function of applying voltageto the power feeding device antenna circuit 701 in accordance with thesignal generated by the signal processing circuit 702 and the signalwith a constant frequency generated by the oscillator circuit 706. Thus,a signal is output from the power feeding device antenna circuit 701. Onthe other hand, when a signal is received from the power receivingdevice antenna circuit 602, the rectifier circuit 703 rectifies thereceived signal. From signals rectified by the rectifier circuit 703,the demodulation circuit 705 extracts a signal transmitted from thepower receiving device 600 to the power feeding device 700. The signalprocessing circuit 702 has a function of analyzing the signal extractedby the demodulation circuit 705.

Note that any circuit may be provided between the circuits as long asthe RF power feeding can be performed. For example, after the powerreceiving device 600 receives a signal and the rectifier circuit 605generates DC voltage, a circuit such as a DC-DC converter or regulatorthat is provided in a subsequent stage may generate constant voltage.Thus, overvoltage application to the inside of the power receivingdevice 600 can be suppressed.

The lithium-ion secondary battery according to one embodiment of thepresent invention is used as the lithium-ion secondary battery 604included in the power receiving device 600 in the RF power feedingsystem shown in FIG. 9.

When the lithium-ion secondary battery according to one embodiment ofthe present invention is used in the RF power feeding system, thedischarge capacity or the charge capacity can be increased as comparedwith the case of using a conventional secondary battery; therefore, thetime interval between wireless power feeding and the next wireless powerfeeding can be longer (power feeding can be less frequent).

In addition, with the use of the lithium-ion secondary battery accordingto one embodiment of the present invention in the RF power feedingsystem, the power receiving device 600 can be compact and lightweight ifthe discharge capacity or the charge capacity with which the power loadportion 610 can be driven is the same as that of a conventionalsecondary battery. Therefore, the total cost can be reduced.

Note that when the lithium-ion secondary battery according to oneembodiment of the present invention is used in the RF power feedingsystem and the power receiving device antenna circuit 602 and thelithium-ion secondary battery 604 overlap with each other, it ispreferred that the impedance of the power receiving device antennacircuit 602 is not changed because of deformation of the lithium-ionsecondary battery 604 due to charge and discharge of the lithium-ionsecondary battery 604 and deformation of an antenna due to the abovedeformation. If the impedance of the antenna is changed, in some cases,electric power is not supplied sufficiently. For example, thelithium-ion secondary battery 604 may be packed in a battery pack formedof metal or ceramics. Note that in that case, the power receiving deviceantenna circuit 602 and the battery pack are preferably separated fromeach other by several tens of micrometers or more.

In this embodiment, the signal for charge has no limitation on itsfrequency and may have any hand of frequency with which electric powercan be transmitted. For example, the signal for charge may have any ofan LF band of 135 kHz (long wave), an HF band of 13.56 MHz (short wave),a UHF band of 900 MHz to 1 GHz (ultra high frequency wave), and amicrowave band of 2.45 GHz.

A signal transmission method may be properly selected from variousmethods including an electromagnetic coupling method, an electromagneticinduction method, a resonance method, and a microwave method. In orderto prevent energy loss due to foreign substances containing moisture,such as rain and mud, an electromagnetic induction method or a resonancemethod using a low frequency band, specifically, frequencies of shortwaves of 3 MHz to 30 MHz, frequencies of medium waves of 300 kHz to 3MHz, frequencies of long waves of 30 kHz to 300 kHz. or frequencies ofultra long waves of 3 kHz to 30 kHz, is preferably used.

This embodiment can be implemented in combination with any of the aboveembodiments.

REFERENCE NUMERALS

-   101: positive electrode current collector, 103: graphene, 105:    lithium-containing composite oxide, 105 a: lithium-containing    composite oxide, 107: binder, 108: gap, 109: slurry, 113: graphene,    115: lithium-containing composite oxide, 117: binder, 118: gap, 121:    positive electrode active material layer, 123: graphene, 125:    lithium-containing composite oxide, 127: binder, 133: graphene, 135:    lithium-containing composite oxide, 141: positive electrode active    material layer, 147: binder, 149: slurry, 157: binder, 161: positive    electrode active material layer, 167: binder, 187: binder, 301: unit    cell, 303: lithium, 305: iron, 307: phosphorus, 309: oxygen, 400:    lithium-ion secondary battery, 401: positive electrode current    collector, 403: positive electrode active material layer, 405:    positive electrode, 407: negative electrode current collector, 409:    negative electrode active material layer, 411: negative electrode,    413: separator, 417: external terminal, 419: external terminal, 421:    gasket, 500: automobile, 502: lithium-ion secondary battery, 600:    power receiving device, 601: power receiving device portion, 602:    power receiving device antenna circuit, 603: signal processing    circuit, 604: lithium-ion secondary battery, 605: rectifier circuit,    606: modulation circuit, 607: power supply circuit, 610: power load    portion, 700: power feeding device, 701: power feeding device    antenna circuit, 702: signal processing circuit, 703: rectifier    circuit, 704: modulation circuit, 705: demodulation circuit, and    706: oscillator circuit.

This application is based on Japanese Patent Application serial no.2011-068599 filed with the Japan Patent Office on Mar. 25, 2011, theentire contents of which are hereby incorporated by reference.

What is claimed is:
 1. A method for manufacturing an electrode,comprising the steps of: forming a first graphene layer over and incontact with a current collector; applying a slurry comprising aplurality of positive electrode active material particles, a binder anda solvent to the first graphene layer over the current collector; andremoving the solvent in the slurry to form an active material layer,wherein the first graphene layer is formed by using a graphene oxideaqueous solution comprising graphene oxide.
 2. The method formanufacturing an electrode according to claim 1, wherein the grapheneoxide is reduced by heating.
 3. The method for manufacturing anelectrode according to claim 1, wherein the graphene oxide is reduced bya reducing agent.
 4. The method for manufacturing an electrode accordingto claim 3, wherein the reducing agent is hydrazine.
 5. The method formanufacturing an electrode according to claim 1, wherein part of thecurrent collector is not covered with the first graphene layer afterforming the first graphene layer.
 6. The method for manufacturing anelectrode according to claim 1, wherein the plurality of positiveelectrode active material particles comprises a first particle and asecond particle, wherein the current collector is in direct contact withthe first particle after forming the active material layer, and whereina piece of graphene in the first graphene layer is in contact with thesecond particle after forming the active material layer.
 7. The methodfor manufacturing an electrode according to claim 1, further comprisingthe step of: forming a second graphene layer over the active materiallayer.
 8. A method for manufacturing an electrode, comprising the stepsof: forming a first graphene layer over and in contact with a currentcollector; applying a slurry comprising a plurality of positiveelectrode active material particles, a binder and a solvent to the firstgraphene layer over the current collector; and removing the solvent inthe slurry to form an active material layer, wherein the first graphenelayer is formed by using graphene, and wherein the graphene is formed byseparation of graphite in a polar solvent.
 9. The method formanufacturing an electrode according to claim 8, wherein part of thecurrent collector is not covered with the first graphene layer afterforming the first graphene layer.
 10. The method for manufacturing anelectrode according to claim 8, wherein the plurality of positiveelectrode active material particles comprises a first particle and asecond particle, wherein the current collector is in direct contact withthe first particle after forming the active material layer, and whereina piece of graphene in the first graphene layer is in contact with thesecond particle after forming the active material layer.
 11. The methodfor manufacturing an electrode according to claim 8, further comprisingthe step of: forming a second graphene layer over the active materiallayer.